This invention relates to a metal material for sliding surfaces to be used in combination with a composite plated sliding surface beyond which coprecipitates project outward.
As a material adapted to slide on a composite plated layer, there has heretofore been used a cast iron consisting of flaky, massive or globular graphite uniformly distributed in a pearlite matrix. Metal materials subjected to a surface treatment, that is, hard chrome plating, metal molybdenum spraying, bronze plating, and the like have also been used. Although the modulus of elasticity, transverse rupture strength, hardness, impact resistance and fatique limit of these metal materials depend on the base materials, there has been a problem from a practical standpoint in that, no matter which material is used, a large amount of wear to both or either one of the composite plated layer and the metal material results. In addition, some of these metal materials have the drawback of damaging the composite plated layer or adhering thereto by fusion seizure. For this reason, in order to utilize composite plated layers, there has been an urgent demand for developing metal materials without any such problems as are mentioned above.
There have been proposed prior arts relating to composite electroplating as typically disclosed, for instance, by a Japanese Pat. No. 280,939. According to this patent, there is disclosed a method wherein there is electrolytically coprecipitated a composite film consisting of any one of metals such as nickel, chromium, copper, cadmium, and optionally selected powdered substances such as alminum oxide, iron oxide, silicon carbide, tungsten carbide, diamond, etc. on the surface of a metal matrix, thereby providing the metal surface with extraordinary resistance against wear, heat, corrosion, etc.. Among these substance applied in such composite electrolytic operation, it has been most common for those skilled in the art to use nickel as a metallic component plus silicon carbide as a powdered admixture.
In this typical application, taken by way of example from among other known arts, wherein a composite film is electroplated, an electrolytic bath containing nickel sulfamate, nickel chloride, boric acid and other effective admixtures is first prepared in an electrolytic cell, thereafter powdered silicon carbide is further dispersed evenly to be suspended therein. In this particular electrolytic bath, it is essential to use caution to keep the powdered substance admixed from settling in the bath. Electrolytic nickel is used as anode and a metallic material to be coated is used as cathode. With such arrangement, electrolytic operation is performed by passing a current through the electrolytic cell and then coprecipitated on the surface of said material is powdered silicon carbide to be evently dispersed in the electroplated layer of nickel. The silicon carbide which is a coprecipitate has an average particle diameter of about 4 .mu.. The silicon carbide content in the composite plated layer is about 6% by weight. The silicon carbide which is a coprecipitate in the composite plating is a hard substance having a knoop hardness of 2480, whereby the hardness of the composite plated layer is enhanced and the wear resistance thereof is improved by coprecipitation of silicon carbide. For the sliding surfaces of machine tools, the inner wall surfaces of cylinders of internal combustion engines, clutch plates, and like parts, a hard metal or a hard metal which has been subjected to a surface treatment is used since the metal wears out through the sliding on, colliding with or being passed by another substance.
There are two types of wear, namely, initial wear that occurs before the two sliding materials are mutually run in, and stationary wear that occurs thereafter. Maximum quantity of wear occurs at a part where the direction of movement is reversed or a lubricating oil film is interrupted. A satisfactory material having high wear resistance means a material having excellent oil film retentivity. This is why cast iron is frequently used as a wear resisting material. More particularly, although cast iron shows slightly different physical properties depending on the form and quantity of graphite precipitated in the structure, cast iron is capable of storing oil to prevent the interruption of oil supply while it serves as a solid lubricant, and recesses from which the graphite have been removed act as oil sumps which held the formation and retention of lubricating oil films on the sliding surfaces. Further, cast iron effectively dissipates frictional heat due to its high thermal conductivity and readily adaptes to fit the form of the other material due to the low modulus of elasticity.
In order to improve the wear resistance of a composite plated layer, it is important to enhance the oil film retentivity as in the case of cast iron. In nickel-silicon carbide composite plating, phosphorus compounds are added in the plating bath to increase the hardness of the plated layer, thereby preventing silicon carbide from being forced into the nickel matrix by striking action or sliding pressure, while, at the same time, the plated layer is electropolished by connecting the plated layer as the anode, thereby causing silicon carbide particles to project beyond the composite plated surface. These particles form the first sliding surface and the recessed surface of the nickel matrix form the second sliding surface with oil being retained between the first and second surfaces, whereby the lubricating performance of the sliding surfaces is improved. The composite plated layer obtained by adding phosphorus compounds to increase the hardness and electropolishing said layer to cause silicon carbide grains to project beyond the surface by from 1 to 2 .mu.(hereinafter referred to as a composite electroplated layer) has higher wear resistance than that of an ordinary composite plated layer.
When this composite electroplated layer is applied to the inner wall surface of a cylinder of an internal combustion engine, the piston rings to slide therealong must have a high wear resistance which is similar to the composite electroplated layer. Since the mechanism of wear and lubrication of a piston-ring material is similar to that of a cylinder material, the composite electroplated layer is also applied to the piston-ring material thereby to improve its wear resistance. As a piston-ring material, pearlitic cast iron containing a large quantity of phosphorus and hard steadite distributed therein with globular graphite uniformly distributed is excellent in wear resistance, thermal resistance, and strength.
When a composite electroplated surface slides on a cast iron surface, the surface of the cast iron is scratched by the hard silicon carbide particles projecting outward beyond the composite electroplated surface, whereby wear is caused. Further, with respect to the cylinder, a lubricating oil film will be interrupted near the top dead center and the bottom dead center where the direction of movement changes, whereby wear will be caused also on the composite electroplated layer. Even cast iron cannot prevent wear, so that a metal material with higher hardness than that of cast iron provided with appropriate oil sumps or space becomes necessary.
The processes for surface treatment of piston rings include parkerizing, hard chrome plating, application of solid lubricants, molybdenum spraying, wet plating, and bronze plating applied on a chrome plated layer, but each of these treatment processes has advantages and disadvantages. Among these processes, hard chrome plating is commonly applied at present for improving durability. A hard chrome plated layer has various advantageous features such as high thermal conductivity, high corrosion resistance, and high melting point rendering the layer difficult to seize by function, and, furthermore, the wear coefficient thereof is sufficiently low. That is, the hardness of the layer is extremely high and the wear resistance thereof is substantially improved, so that the hard chrome plated layer, when applied to a piston ring, can greatly extend the life of the ring. However, the piston ring surface thus plated also has disadvantages. For example, it does not easily assume an intimately run-in state relative to the cylinder, and the cylinder wear becomes severe.